1. Field of the Invention
The present invention relates to an improvement in a process for producing carbonyl compounds having at least 6 carbon atoms which comprises oxidizing a corresponding olefinic compound in aqueous medium containing palladium or salt thereof and a copper salt.
2. Description of the Prior Art
A method of producing carbonyl compounds by contacting olefinic compounds with aqueous medium containing palladium or its salt as a main catalyst component is well known as aqueous Wacker oxidations (for example, U.S. Pat. Nos. 3,122,586 and 3,154,586). An industrial application of this reaction is now seen in the production of acetaldehyde and acetone from ethylene and propylene respectively. Said oxidation is in general carried out in the presence of a redox promotor as a co-catalyst represented by a copper salt in view of effective utilization of such an expensive palladium catalyst. This reaction is carried out, in a gaseous atmosphere with or without oxygen. The latter case is called the "two step method", because there is required an additional step to reoxidize the reduced catalyst component with oxygen-containing gas for reuse of said component. On the other hand the former is called the "single step method", because the reoxidation step is not always necessary. An advantage of the two step method is that undesirable side reactions due to oxygen can be avoided and therefore it is in particular preferably applicable in cases where the reaction products (e.g. aldehydes) are chemically unstable to oxygen.
An olefinic compound having at least 6 carbon atoms in the molecule is relatively insoluble in water. Accordingly, for the purpose of producing the corresponding carbonyl compound from said olefinic compound by the abovementioned reaction, it has been considered necessary to (1) dissolve the olefinic compound in the reaction system by using water-soluble polar solvents such as dimethylformamide, dimethylacetamide, hexamethylphosphoric triamide, dimethylsulfoxide, sulfolane, dioxane or tetrahydrofuran in combination with water [J. Org. Chem., 29, 241 (1964) and 39, 3276 (1974)] or (2) disperse the olefinic compound in water by adding a suitable amount of cationic or nonionic surfactant to the reaction system [Tetrahedron Letters, 3817-3820 (1977)]. When industrial operation is intended, however, it may be poined out that such a reaction is accompanied with enormous troubles. Thus, the above method (1) involves the following problems: (a) in the case that the two step method is adopted, palladium metal partially precipitates and deposits on the wall of a reactor in the oxidation, whereby a problem on process engineering takes place; (b) the reaction system becomes chemically unstable, because many water-soluble polar solvents are susceptible to partial hydrolysis under oxidative conditions; (c) a lot of steam is required due to the presence of a polar solvent and such a solvent is contaminated into the distillate in separating and recovering the product by steam distillation; (d) solvent-extraction for the separation of the product requires a large amount of the extracting solvent, and the distribution coefficient of such a polar solvent and catalyst component into the extracting layer is so high that separation of the product and recycle of the catalyst become troublesome. Further, the above method (2) involves the following problems: (a) deposition of metallic palladium on the reactor wall is apt to take place when the two step method is employed; (b) foaming is apt to take place in separation of the product by steam distillation; (c) a vague interface is observed between an aqueous layer and an organic layer in extracting the product with a solvent; and (d) the reaction velocity is lower than in the method (1).